Bake the chocolate cake at 350°F until the center springs back, then let it cool completely.
Cut the cooled cake into 1-inch cubes; chill briefly if the cubes crumble.
Whip the heavy cream with powdered sugar and vanilla until stiff peaks form; fold in a splash of cream if over-whipped.
Drain or pit the cherries, mix with ¼ cup cherry juice, and let them rest for 10 minutes.
Begin layering with cake cubes, pudding, cherries with a little juice, and whipped cream.
Repeat the layers in the same order until reaching the top, finishing with whipped cream.
Garnish with dark chocolate shavings, whole cherries, and optional cocoa powder.
Chill the trifle for at least 3 hours or overnight to allow the flavors to meld.